Planet Gliese 581C, which is 1.5 times larger and five times more massive than Earth, could be covered in oceans - and support life.

It orbits in a region around parent star Gliese 581 classed as a habitable zone.

Dr Xavier Delfosse, a member of the team from Grenoble University that made the discovery, said: "Liquid water is critical to life as we know it. On the treasure map of the universe, one would be tempted to mark this planet with an X."

Gliese 581c is the smallest extra-solar planet yet found. The majority of planets discovered outside the Solar System are giant gaseous worlds the size of Jupiter or bigger.

Life as we know it could not exist on them. But the new planet is unusual because it is so small, and therefore probably rocky.

It is estimated to have double Earth's gravity and likely to have an atmosphere. Two other planets also orbit Gliese 581, which lies in the constellation of Libra and is among the 100 closest stars to the Sun.

The planet was found by Swiss, French and Portuguese astronomers using the European Southern Observatory at La Silla in Chile.

Dr Seth Shostak of the Search for Extra-Terrestrial Intelligence Institute in the US said: "I am excited about this discovery. We assumed small planets will exist in great numbers, and some by chance will be in the habitable zone."

He added that since Gliese 581 is much older than the Sun, life on the planet may have existed for longer than on Earth and could therefore be more advanced.

Dr Shostak said: "Life on Earth is four billion years old and it's taken all of that to produce us. The older the star, maybe the greater the chance it has produced something clever."